The present invention relates to a controlled current inverter induction motor drive and more particularly to a controlled current inverter induction motor drive that controls motor current and the angle between motor flux and motor current.
Previous current controlled inverter induction motor drives, such as the one shown in Espelage et al. U.S. Pat. No. 4,230,979, filed Apr. 10, 1978 and assigned to the same assignee as the present invention, use a control based on DC-link current and the angle between motor flux and motor current. The motor current to flux angle is determined indirectly by calculating the sine of the motor current/flux angle using two squaring circuits, one square root circuit, three multiplier circuits and a divider circuit which makes the analog implementation complex and costly. The calculated sin .theta. is used as a feedback signal to compare to a commanded sin .theta. angle. Furthermore, the load thyristor firing is controlled by the output of a load angle regulator (sin .theta.) supplying a voltage controlled oscillator which then supplies a six stage ring counter, which in combination with additional logic, sequentially gates load inverter thyristors.
It is an object of the present invention to control the motor current/flux angle in an induction motor drive in a direct manner simplifying the implementation without sacrificing performance.
It is a further object of the present invention to control an induction motor drive by controlling motor current and the angle between motor flux and motor current where this angle is derived directly from a phase-locked loop synchronized to integrated motor voltage.